LONG-WAVE RADIO TRANSMISSION PHENOMENA 11 



decrease in sky-wave attenuation beginning shortly before tangential 

 sunset and continuing for from one-half to two hours after sunset. 

 On the 60-kc. long-distance path the evidence of the initial phases of 

 this phenomenon is not so well defined, due probably to the gradual 

 nature of the change, and to the characteristic sunset minimum which 

 occurs shortly after sunset. Recent measurements made on the 

 transatlantic radio telephone channels, however, indicate a presunset 

 rise in field of about 2 db, beginning when the sun's altitude becomes 

 low enough to cause an appreciable lengthening of the atmospheric 

 ray path, thereby decreasing the intensity of the active solar rays. A 

 presunset increase is very apparent on the Rocky Point-Houlton 60-kc. 

 measurements, beginning at about 40 minutes before, and continuing 

 to a maximum at the instant of mid-path sunset. There is a possi- 

 bility, however, that this phase may be due to interference phenomena. 



More conclusive evidence of the effect of a reduction in the intensity 

 of the sun's ionizing rays is provided by long-wave field strength 

 measurements made during the solar eclipses of January 24, 1925, and 

 August 31, 1932. The data taken in 1925 were secured by means of an 

 automatic recorder on the Rocky Point-Belfast 57-kc. path and manual 

 measurements were made at the European receiving stations. In 

 1932, automatic recorders were used on both the Rocky Point-Houlton 

 60-kc. path and the Rugby-Houlton 68-kc. path. In all cases where 

 automatic recorders were used the data were abstracted from the 

 record and replotted for reproduction. 



Figures 4 and 5 show the eclipse circumstances and the concurrent 

 variations in the measured radio field strength. The Chedzoy, New 

 Southgate, and somewhat less clearly the Aberdeen measurements in 

 1925, show the results of reduced attenuation almost immediately after 

 the first darkening of the transmission path. For the 1932 data this 

 effect is clearly evident on the Rocky Point-Houlton measurements. 



The sudden increase in field to be expected at the beginning of the 

 eclipse at Belfast in 1925 is not apparent from the data, and its absence 

 may be due either to improper recorder operation or to some fortuitous 

 phenomenon peculiar to that particular eclipse, such as, for example, 

 the possibility that the phases of ground and sky waves were in 

 quadrature. The Rugby-Houlton observations in 1932 likewise are 

 ambiguous because the true eclipse effect is complicated by superposed 

 sunset effects originating at the eastern path terminal, and the ob- 

 served increase in field may be due to these rather than to the eclipse. 



In all cases except the two mentioned above, the increase in field 

 was followed by a rapid drop, with a minimum occurring at the 

 approximate time the totality shadow crossed the transmission path. 



